Internet DRAFT - draft-landau-acme-caa
draft-landau-acme-caa
ACME Working Group H. Landau
Internet-Draft April 21, 2016
Intended status: Informational
Expires: October 23, 2016
ACME Account Key Binding via CAA Records
draft-landau-acme-caa-00
Abstract
The ACME protocol provides a means for hosts to automatically request
and obtain X.509 certificates from certificate authorities.
Certification authorities which implement ACME may also choose to
implement the CAA DNS record, which allows a domain to communicate
issuance policy to CAs. The CAA specification alone allows a domain
to define policy with CA-level granularity. However, the CAA
specification also provides facilities for extension to admit more
granular, CA-specific policy. This specification defines such a
parameter.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 23, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Extensions to the CAA Record . . . . . . . . . . . . . . . . 2
3.1. acme-ak Parameter . . . . . . . . . . . . . . . . . . . . 2
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
4.1. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Authorization Freshness . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . 4
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
This specification defines a parameter for the 'issue' and
'issuewild' properties of the Certification Authority Authorization
(CAA) DNS resource record [RFC6844], allowing authorization conferred
by a CAA policy to be restricted to specific ACME
[I-D.ietf-acme-acme] accounts. The accounts are identified by
account key thumbprint.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for compliant ACME-CAA
implementations.
3. Extensions to the CAA Record
3.1. acme-ak Parameter
A CAA parameter "acme-ak" is defined for the 'issue' and 'issuewild'
properties defined by [RFC6844]. The value of this parameter, if
specified, MUST be the base64url [RFC4648] encoding (without padding)
of the JWK thumbprint [RFC7517] of the ACME account key
[I-D.ietf-acme-acme].
If an ACME server finds multiple CAA records pertaining to it (i.e.,
having property 'issue' or 'issuewild' as applicable and a domain
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that the ACME server recognises as its own) with different "acme-ak"
parameters, the ACME server MUST NOT consider the CAA record set to
authorize issuance unless at least one of the specified account key
thumbprints matches the requesting ACME account key. A property
without an "acme-ak" parameter matches any account key. A property
with an invalid "acme-ak" parameter (i.e. not 43 characters long or
not a valid base64url string), or multiple "acme-ak" parameters is
unsatisfiable.
4. Security Considerations
This specification describes an extension to the CAA record
specification increasing the granularity at which CAA policy can be
expressed for ACME-based CAs. This allows the set of entities
capable of successfully requesting issuance of certificates for a
given domain to be restricted beyond that which would otherwise be
possible, while still allowing issuance for specific ACME account
keys. This improves the security of issuance for domains which
choose to employ it, when combined with a CA which implements this
specification.
4.1. DNSSEC
Where a domain chooses to secure its nameservers using DNSSEC, the
authenticity of an ACME account key nomination placed in a CAA record
can be assured, providing that a CA makes all DNS resolutions via an
appropriate, trusted DNSSEC-validating resolver. In this case and so
long as control of nominated keys is retained, a domain is protected
from the threat posed by a global adversary capable of performing
man-in-the-middle attacks, which could otherwise forge DNS responses
and successfully obtain ACME authorizations and certificates for the
domain.
4.2. Authorization Freshness
The CAA specification governs the act of issuance by a CA. The act
of authorization as described by the ACME protocol occurs separately
to issuance and may occur substantially prior to an issuance request.
The CAA policy expressed by a domain may have changed in the
meantime, creating the risk that a CA will issue certificates in a
manner inconsistent with the presently published CAA policy.
CAs SHOULD consider adopting practices to reduce the risk of such
circumstances. Possible countermeasures include issuing ACME
authorizations with very limited validity periods, such as an hour,
or revalidating the CAA policy for a domain at certificate issuance
time.
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5. IANA Considerations
None. As per the CAA specification, the parameter namespace for the
CAA 'issue' and 'issuewild' properties has CA-defined semantics.
This document merely specifies a RECOMMENDED semantic for a parameter
of the name "acme-ak" for ACME-based CAs.
6. Normative References
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf-
acme-acme-02 (work in progress), March 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
<http://www.rfc-editor.org/info/rfc6844>.
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/
RFC7517, May 2015,
<http://www.rfc-editor.org/info/rfc7517>.
Appendix A. Examples
The following shows an example DNS configuration which nominates two
account keys as authorized to issue certificates for the domain
"example.com". Issuance is restricted to the CA "example.net".
example.com. IN CAA 0 issue "example.net; \
acme-ak=UKNmi2whPhuAhDvAxGa_aOZgPzyJDhhsrt-8Bt2fWh0"
example.com. IN CAA 0 issue "example.net; \
acme-ak=rlp4OZPOR9MKejkOdZAKQ5Tfwce6llawmrDIh-BtNJ0"
Author's Address
Hugo Landau
Email: hlandau@devever.net
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